Linear polyesters, for example, polyethylene terephthalate or polybutylene terephthalate, are being utilized to an increasing extent not only for the production of fibers but also for the manufacture of sheets and molded articles. A fundamental disadvantage of these polyesters is that they support combustion, especially in the molten condition. Therefore, for many practical applications, it is necessary to render these polymers nonflammable.
Three basic methods are known for imparting flame resistance to linear polyesters:
(1) TREATMENT OF THE SURFACE WITH APPROPRIATE COMPOUNDS;
(2) INCORPORATION OF FLAME-RESISTANT FUNCTIONS INTO THE POLYESTER CHAIN BY CONDENSATION, OR
(3) PHYSICAL INCORPORATION OF FLAME RETARDANTS INTO THE POLYESTER BY MIXING.
The first two possibilities are of little advantage, for the reasons that the treating agent is retained on the final product for only a limited time period, or the properties of the linear polyester are adversely affected to a great extent. The third method is most advantageous, since flame retardants can be permanently incorporated into the final products without altering the properties of the linear polyesters excessively.
Thus, DOS (German Unexamined Laid-Open Application) No. 2,162,437 describes admixing bromine-containing aromatics and aryl phosphates with linear polyesters. A similar combination is described in DOS No. 2,139,395. In the latter disclosure, a mixture consisting of brominated diphenyls and triphenylphosphine oxide is used. A disadvantage of the foregoing combinations is that brominated aromatic compounds contained therein have a strong tendency to sublime at processing temperatures required for linear polyesters. Medical investigations found that the brominated compounds are difficult to degrade biologically and accumulate preferentially in liver tissue. This presents hazards which greatly restrict industrial use of these compounds.
It is an object of this invention to provide nonflammable molding compositions of linear polyesters which do not exhibit any of the above-described disadvantages.